On Xiaomi Redmi Note 4 with MIUI 10, Android 7 there is no way to enable sound for calendar notifications, I get vibrations only. I have tried several calendar apps including Google, MIUI, aCalendar+, various settings including setting battery optimisation off but it never plays a sound, it only vibrates.
When I press TEST in aCalendar+ notifications settings it vibrates but makes no sound and provides no further help. Of course my sounds are on and I have even tried to turn off bluetooth. I have no SD card, all sounds are stored in internal memory.
Notifications form WhatsApp or Facebook messenger work well with the sound.